Common Predefined Macros

The common predefined macros are GNU C extensions. They are available
with the same meanings regardless of the machine or operating system on
which you are using GNU C. Their names all start with double
underscores.

__GNUC__

__GNUC_MINOR__

__GNUC_PATCHLEVEL__

These macros are defined by all GNU compilers that use the C
preprocessor: C, C++, and Objective-C. Their values are the major
version, minor version, and patch level of the compiler, as integer
constants. For example, GCC 3.2.1 will define __GNUC__ to 3,
__GNUC_MINOR__ to 2, and __GNUC_PATCHLEVEL__ to 1. They
are defined only when the entire compiler is in use; if you invoke the
preprocessor directly, they are not defined.

__GNUC_PATCHLEVEL__ is new to GCC 3.0; it is also present in the
widely-used development snapshots leading up to 3.0 (which identify
themselves as GCC 2.96 or 2.97, depending on which snapshot you have).

If all you need to know is whether or not your program is being compiled
by GCC, you can simply test __GNUC__. If you need to write code
which depends on a specific version, you must be more careful. Each
time the minor version is increased, the patch level is reset to zero;
each time the major version is increased (which happens rarely), the
minor version and patch level are reset. If you wish to use the
predefined macros directly in the conditional, you will need to write it
like this:

GCC defines this macro if and only if the -ansi switch, or a
-std switch specifying strict conformance to some version of ISO C,
was specified when GCC was invoked. It is defined to 1.
This macro exists primarily to direct GNU libc's header files to
restrict their definitions to the minimal set found in the 1989 C
standard.

__BASE_FILE__

This macro expands to the name of the main input file, in the form
of a C string constant. This is the source file that was specified
on the command line of the preprocessor or C compiler.

__INCLUDE_LEVEL__

This macro expands to a decimal integer constant that represents the
depth of nesting in include files. The value of this macro is
incremented on every #include directive and decremented at the
end of every included file. It starts out at 0, it's value within the
base file specified on the command line.

__VERSION__

This macro expands to a string constant which describes the version of
the compiler in use. You should not rely on its contents having any
particular form, but it can be counted on to contain at least the
release number.

__OPTIMIZE__

__OPTIMIZE_SIZE__

__NO_INLINE__

These macros describe the compilation mode. __OPTIMIZE__ is
defined in all optimizing compilations. __OPTIMIZE_SIZE__ is
defined if the compiler is optimizing for size, not speed.
__NO_INLINE__ is defined if no functions will be inlined into
their callers (when not optimizing, or when inlining has been
specifically disabled by -fno-inline).

These macros cause certain GNU header files to provide optimized
definitions, using macros or inline functions, of system library
functions. You should not use these macros in any way unless you make
sure that programs will execute with the same effect whether or not they
are defined. If they are defined, their value is 1.

__CHAR_UNSIGNED__

GCC defines this macro if and only if the data type char is
unsigned on the target machine. It exists to cause the standard header
file limits.h to work correctly. You should not use this macro
yourself; instead, refer to the standard macros defined in limits.h.

__REGISTER_PREFIX__

This macro expands to a single token (not a string constant) which is
the prefix applied to CPU register names in assembly language for this
target. You can use it to write assembly that is usable in multiple
environments. For example, in the m68k-aout environment it
expands to nothing, but in the m68k-coff environment it expands
to a single %.

__USER_LABEL_PREFIX__

This macro expands to a single token which is the prefix applied to
user labels (symbols visible to C code) in assembly. For example, in
the m68k-aout environment it expands to an _, but in the
m68k-coff environment it expands to nothing.

This macro will have the correct definition even if
-f(no-)underscores is in use, but it will not be correct if
target-specific options that adjust this prefix are used (e.g. the
OSF/rose -mno-underscores option).

__SIZE_TYPE__

__PTRDIFF_TYPE__

__WCHAR_TYPE__

__WINT_TYPE__

These macros are defined to the correct underlying types for the
size_t, ptrdiff_t, wchar_t, and wint_t
typedefs, respectively. They exist to make the standard header files
stddef.h and wchar.h work correctly. You should not use
these macros directly; instead, include the appropriate headers and use
the typedefs.

__USING_SJLJ_EXCEPTIONS__

This macro is defined, with value 1, if the compiler uses the old
mechanism based on setjmp and longjmp for exception
handling.